The invention relates to a process and an apparatus for the measurement of small quantities of light.
Processes and devices of this type are known and are used for example in photographic printers, wherein the copy master, consisting for example of positive or negative films, is measured in order to determine the quantities of copying light required for the subsequent exposure of the copy material, for example photographic paper. The films are measured for example in a manner such that the film is exposed in numerous individual measuring points with light, which penetrates the film to be measured. The light transmitted through the film is received and the transmissivity of the film determined by the quantity of light transmitted through each measuring point. The light quantities are usually determined with the aid of photodiodes which convert the light received into an electrical signal corresponding to the quantity of light received.
The trend in measuring copy masters (films) is toward increasingly higher resolutions of the master. In practice, this means that each individual image field is measured in more and more individual measuring points. This renders the quantity of light penetrating the film in the individual measuring points progressively smaller. Consequently, the electrical signal produced by the photodiode (photo current) will also be smaller. Additionally, the measuring intervals, i.e., the measuring times, wherein the individual measuring points are measured, are becoming shorter.
The amplification of the outlet signal by an operational amplifier (OpAmp circuit) which contains a resistance in its feedback loop, raises certain problems. An amplifier circuit of this type must be provided additionally with a capacitor parallel to the feedback resistance, so that the unavoidable capacitance of the photodiode (blocking layer capacitance, etc.) may be compensated, thereby avoiding the overshooting of the outlet signal of the amplifier circuit. Another reason for such a capacitor in the feedback loop is that in the finite amplification bandwidth of the operational amplifier, as in this manner, a defined frequency variation is produced over the bandwidth to be measured.
However, there is a disadvantage that the noise current caused by the resistance as a function of the size of the resistance and the signal bandwidth to be measured, is large enough so that in the case of acceptable signal amplifications (i.e., with a large feedback resistance) usable signal-noise ratios can no longer be obtained. Since the capacitance of the feedback capacitor must be maintained small, the response time of the OpAmp circuit can be affected only by the size of the feedback resistance. However, the response time must be very small as in the case of high resolutions of the master, numerous measuring points must be measured during a brief period of time and the outlet signal must be responding during this time. Consequently, the feedback resistance must be chosen as small as possible, which in turn renders the noise current generated by it large enough so that practically no usable signal-noise ratio may be obtained, or the measuring signal cannot even be recognized separately from the noise. Furthermore, this consideration does not even take into account the residual noise of the operational amplifier (OpAmp) itself, which is added to the resistance noise.
An amplifier circuit is thus proposed, which is not limited by the detrimental properties of such resistances. For this purpose, only one capacitor is provided in the feedback loop of the amplifier circuit. The outlet signal of this amplifier circuit (integrator) is then a measuring signal that is proportional not to the photo current, but to the time integral of it, i.e., proportional to the charge generated in the photodiode. However, at the onset of a measurement the capacitor must be discharged to create an initial condition that is independent from the previous history of the circuit in order to prevent any falsification of the measured result by residual energies remaining in the capacitor. This discharge of the capacitor is effected by means of a MOSFET switch. But due to the unavoidable switching capacities of such a MOSFET switch (Gate-Drain-capacitance, etc.) a charge is transported to the inlet of the amplifier circuit, causing a large offset at the outlet of the amplifier circuit. This offset is not the same in each switching process, i.e., there are certain fluctuations of the offset, and so-called switching noise. There are methods known to largely eliminate this offset. However, switching noise cannot be eliminated in this manner so that particularly in the case of small quantities of light to be measured and the corresponding small electrical signals of the photodiodes and also the short integration times due to the large number of measuring points and thus small measuring signals, in this fashion again practically no reliable signal may be produced for the quantity of light received.
It is therefore an object of the invention to make possible the measurement of small quantities of light and the corresponding small measuring signals of the photodiode, which would permit the reliable measurement of light quantities received by a receiver and in particular a photodiode, even in the presence of such switching noises. This should be possible even with short measuring times, such as those required in the case of high resolutions and the corresponding numerous measuring points of the master to be measured. In addition, the measurements should be independent of previous histories to avoid any falsification of the measured results.